In recent years, fuel cell powered vehicles that run on hydrogen as fuel, and hydrogen stations where fuel cell powered vehicles are supplied with hydrogen, have been under development. A stainless steel is one of the candidates for a material used for fuel cells.
When a stainless steel is used for fuel cells, the stainless steel is used in a high-pressure hydrogen gas environment. For this reason, a stainless steel used for fuel cells needs to have an excellent hydrogen brittleness resistance. At present, according to the standards of compressed hydrogen containers for automobiles provided by the High Pressure Gas Safety Act, SUS316L is accredited as a stainless steel having an excellent hydrogen brittleness resistance.
However, in consideration of lowering weight of fuel cell powered vehicles, downsizing hydrogen stations, and operations under high pressure in a hydrogen station, it is preferable that a stainless steel used for the above applications has a high strength.
As previously described, a stainless steel used for fuel cells needs to have an excellent hydrogen brittleness resistance and a high strength. Meanwhile, when a stainless steel is to be used for fuel cells, the stainless steel is processed into a desired shape. For example, machining such as cutting may be performed on stainless steel products of high strength. In this case, it is preferable that the stainless steel further has an excellent machinability.
International Application Publication No. WO2004/083476 (Patent Literature 1), International Application Publication No. WO2004/083477 (Patent Literature 2), International Application Publication No. WO2004/111285 (Patent Literature 3), and International Application Publication No. WO2012/132992 (Patent Literature 4) propose stainless steels that are used in high-pressure hydrogen environments and have high strengths.
The stainless steel for hydrogen gas disclosed in Patent Literature 1 contains: in mass %, C: 0.02% or less; Si: 1.0% or less; Mn: 3 to 30%; Cr: more than 22% to 30%; Ni: 17 to 30%; V: 0.001 to 1.0%; N: 0.10 to 0.50%; and Al: 0.10% or less, with the balance being Fe and impurities. Of the impurities, P is 0.030% or less, S is 0.005% or less, and Ti, Zr, and Hf are each 0.01% or less. The contents of Cr, Mn, and N satisfy the following formula.5Cr+3.4Mn≤500N
The stainless steel for high-pressure hydrogen gas disclosed in Patent Literature 2 contains: in mass %, C: 0.04% or less; Si: 1.0% or less; Mn: 7 to 30%; Cr: 15 to 22%; Ni: 5 to 20%; V: 0.001 to 1.0%; N: 0.20 to 0.50%; and Al: 0.10% or less, with the balance being Fe and impurities. Of the impurities, P is 0.030% or less, S is 0.005% or less, and Ti, Zr, and Hf are each 0.01% or less, which satisfy the following formula.2.5Cr+3.4Mn≤300N
The austenitic stainless steel for hydrogen gas disclosed in Patent Literature 3 has a chemical composition including: in mass %, C: 0.10% or less; Si: 1.0% or less; Mn: 0.01 to 30%; P: 0.040% or less; S: 0.01% or less; Cr: 15 to 30%; Ni: 5.0 to 30%; sol.Al: 0.10% or less; and N: 0.001 to 0.30%, with the balance being Fe and impurities. The austenitic stainless steel includes a micro-structure in which an X-ray integrated intensity I (111) on a cross section along a direction perpendicular to a processing direction is five times or less as much as that in a random orientation, and an X-ray integrated intensity I (220) on a cross section along the processing direction satisfies (220)/I (111)≤10.
The austenitic stainless steel for high-pressure hydrogen gas disclosed in Patent Literature 4 contains: in mass %, C: 0.10% or less; Si: 1.0% or less; Mn: 3% or more and less than 7%; Cr: 15 to 30%; Ni: 10% or more and less than 17%; Al: 0.10% or less; N: 0.10 to 0.50%; and at least one of V: 0.01 to 1.0% and Nb: 0.01 to 0.50%, with the balance being Fe and impurities. Of the impurities, P is 0.0050% or less, and S is 0.050% or less. The austenitic stainless steel contains an alloy carbo-nitride at 0.4/μm2 or more in cross section observation, the alloy carbo-nitride having a tensile strength of 800 MPa or more, a grain size number (ASTM E112) of 8 or more, and a maximum diameter of 50 to 1000 nm.